This invention relates to a method and apparatus for detecting an object.
A prior art method for detecting an object comprises radiating or transmitting a radio wave or sound wave to find the presence or absence of an object, detecting an echo of the radio wave or sound wave reflected from the object when the object is present, and determining the position of the object on the basis of the length of time elapsed from the time of transmission of the wave to the time of detection of the echo or reflected wave. Such a method has been used for detection of either an underground buried object or for detection of a defect in an object.
FIG. 4 shows the structure of a prior art apparatus for detecting an underground buried object by using a radio wave, as disclosed in "Underground radar system" by Ikuo Arai et al, Transactions of the Institute of Electronics and Communication Engineering (Japan), June 1983, Vol. J66-B, No. 6. Referring to FIG. 4, an oscillator 1 oscillates at a high frequency radiates a radio wave toward and into the ground from an antenna 6 through a transmission/reception selector 2 which is changed over to its transmission mode. The wave reflected from a buried object 7 beneath the ground surface 8 is received by the antenna 6, this time as a reflected signal, and is transmitted by the transmission/reception selector 2 while in its reception mode, and is applied through an amplifier 3 to a display unit 5. A control unit 4 computes the depth of the buried object 7 on the basis of the time difference between the time of wave transmission from the oscillator 1 and the time of reflected wave reception and, also, on the basis of the propagation velocity of the radio wave, and determines the point to be displayed on the display unit 5.
For the purpose of detection of the horizontal position of the buried object 7, the antenna 6, which radiates the radio wave and detects the reflected wave, is moved in the horizontal direction. The horizontal distribution of the intensity of the reflected wave and that of the length of time elapsed until detection of the reflected wave are computed to ascertain the horizontal position of the buried object 7 on the basis of the point where the reflected wave intensity is maximum and the reflected wave detection time is minimum.
Part of the radio wave radiated from the antenna 6 is reflected by the ground surface 8, and the remainder propagates into the ground. Part of the radio wave reflected from the ground surface 8 is reflected by the antenna 6, and such reflection between the ground surface 8 and the antenna 6 is repeated until the wave is sufficiently attenuated. The signal attributable to the wave reflection from the ground surface 8 is received until the radio wave is sufficiently attenuated after a period of time of 2h/v sec has elapsed from the time of wave radiation, where h is the distance between the antenna 6 and the ground surface 8 in meters and v is the wave propagation velocity in m/sec. Therefore, there has been such a possibility that the reflected wave from the ground surface 8 is mistaken as the reflected wave from the buried object 7 when judgment is based on only the length of time elapsed from the time of wave radiation to the time of reflected wave reception.
The reflectivity of the ground surface 8 for radio wave is approximately equal to or more than 0.3. On the other hand, the intensity of radio wave penetrating the ground surface 8 and propagating through the earth is greatly attenuated by the conductivity of the earth. When the depth of the buried object 7 from the ground surface 8 is about 1 to 2 meters, the intensity of reflected wave from the buried object 7 becomes lower than that of wave repeatedly reflected from the ground surface 8. The reflected wave from the buried object 7 and that from the ground surface 8 overlap each other in the received signal. Therefore, the prior art apparatus has had a possibility of misjudging the presence or absence, size and depth of the buried object 7.
In addition a so-called synthetic aperture processing method has been employed hitherto as a method of signal processing capable of detecting the position of an object with high accuracy. According to this method, the distance between the antenna 6 and a wave reflecting object is computed on the basis of the length of time elapsed from the time of radio wave radiation to the time of reflected wave reception, and the position of the reflecting object is determined on the basis of the change in the distance between the antenna 6 and the reflecting object due to the movement of the antenna 6. However, it is difficult to accurately determine the position of the buried object 7 when the reflected wave from the ground surface 8 cannot be separated from the reflected wave from the buried object 7 as described above. Further, according to the prior art, in the synthetic aperture processing method, the reflected signal from the buried object 7 cannot be recognized before the step of signal processing. Therefore, it has been necessary to apply the synthetic aperture processing to the time range in which the reflected wave from the ground surface 8 is received only or the time range wherein no reflected wave appears, resulting in an extended period of time required for processing.
The above problem involved in the prior art buried-object detecting apparatus is also encountered in a prior art ultrasonic defect detecting apparatus. FIG. 2, in which the same or like components are shown by the same reference numerals as FIG. 4, shows the structure of such a prior art ultrasonic defect detecting apparatus. Referring to FIG. 2, a pulse signal is applied from an oscillator 1 to a probe 9 moved in the horizontal direction by a driver 10, and ultrasonic wave transmitted from the probe 9 propagates through an object 11 to be tested. The reflected wave from a defect 12 such as a scar, if any, is received by the probe 9, and the position of the wave reflector such as the scar 12 is determined by a control unit 4 on the basis of the length of time elapsed from the time of wave radiation to the time of reflected wave reception. The property of the propagation medium changes at the portion adjacent to the end of the probe 9, and reflection of the ultrasonic wave results. This reflection of the ultrasonic wave corresponds to the reflection of the radio wave from the ground surface 8 in the case of the aforementioned buried-object detecting apparatus. Therefore, the ultrasonic wave repeatedly reflected inside the probe 9 overlaps the reflected wave from a defect 12 existing near the surface of the object 11, and it has been difficult to distinguish the latter wave from the former wave. A problem similar to that described already with reference to the buried-object detecting apparatus has also been encountered when the synthetic aperture processing method is applied to the ultrasonic defect detecting apparatus for signal processing.